178
Chapter 6
are grouped into bundles called nerves. The peripheral nervous
system has 43 pairs of nerves: 12 pairs of cranial nerves and
31 pairs that connect with the spinal cord as the spinal nerves.
Table 6–8
lists the cranial nerves and summarizes the infor-
mation they transmit. The 31 pairs of spinal nerves are des-
ignated by the vertebral levels from which they exit: cervical,
thoracic, lumbar, sacral, and coccygeal (
Figure 6–42
). The
eight pairs of cervical nerves control the muscles and glands
and receive sensory input from the neck, shoulders, arms, and
hands. The 12 pairs of thoracic nerves are associated with the
chest and upper abdomen. The fi ve pairs of lumbar nerves are
associated with the lower abdomen, hips, and legs, and the fi ve
pairs of sacral nerves are associated with the genitals and lower
digestive tract. (A single pair of coccygeal nerves associated
with the tailbone brings the total to 31 pairs.)
These peripheral nerves can contain nerve fi bers that
are the axons of efferent neurons, afferent neurons, or both.
Therefore, fi bers in a nerve may be classifi ed as belonging to
the
efferent
or the
afferent division
of the peripheral ner-
vous system (refer back to Figure 6–37). All the spinal nerves
contain both afferent and efferent fi bers, whereas some of the
cranial nerves (the optic nerves from the eyes, for example)
contain only afferent fi bers.
As noted earlier, afferent neurons convey information
from sensory receptors at their peripheral endings to the cen-
tral nervous system. The long part of their axon is outside
the central nervous system and is part of the peripheral ner-
vous system. Afferent neurons are sometimes called primary
afferents or fi rst-order neurons because they are the fi rst cells
entering the central nervous system in the synaptically linked
chains of neurons that handle incoming information.
Efferent neurons carry signals out from the central ner-
vous system to muscles or glands. The efferent division of the
peripheral nervous system is more complicated than the affer-
ent, being subdivided into a
somatic nervous system
and an
autonomic nervous system.
These terms are somewhat mis-
leading because they suggest the presence of additional ner-
vous systems distinct from the central and peripheral systems.
Keep in mind that these terms together make up the efferent
division of the peripheral nervous system.
Table 6–8
The Cranial Nerves
Name
Fibers
Comments
I. Olfactory
Afferent
Carries input from receptors in olfactory (smell) neuroepithelium. Not a true nerve.
II. Optic
Afferent
Carries input from receptors in eye. Not a true nerve.
III. Oculomotor
Efferent
Innervates skeletal muscles that move eyeball up, down, and medially and raise upper
eyelid; innervates smooth muscles that constrict pupil and alter lens shape for near
and far vision.
Afferent
Transmits information from receptors in muscles.
IV. Trochlear
Efferent
Innervates skeletal muscles that move eyeball downward and laterally.
Afferent
Transmits information from receptors in muscles.
V. Trigeminal
Efferent
Innervates skeletal chewing muscles.
Afferent
Transmits information from receptors in skin; skeletal muscles of face, nose, and
mouth; and teeth sockets.
VI. Abducens
Efferent
Innervates skeletal muscles that move eyeball laterally.
Afferent
Transmits information from receptors in muscles.
VII. Facial
Efferent
Innervates skeletal muscles of facial expression and swallowing; innervates nose,
palate, and lacrimal and salivary glands.
Afferent
Transmits information from taste buds in front of tongue and mouth.
VIII. Vestibulocochlear
Afferent
Transmits information from receptors in ear.
IX. Glossopharyngeal
Efferent
Innervates skeletal muscles involved in swallowing and parotid salivary gland.
Afferent
Transmits information from taste buds at back of tongue and receptors in auditory-
tube skin.
X. Vagus
Efferent
Innervates skeletal muscles of pharynx and larynx and smooth muscle and glands of
thorax and abdomen.
Afferent
Transmits information from receptors in thorax and abdomen.
XI. Accessory
Efferent
Innervates neck skeletal muscles.
XII. Hypoglossal
Efferent
Innervates skeletal muscles of tongue.
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